(1) Field of the Invention
This invention relates to a novel electrical insulating oil and oil-filled electrical appliances which is impregnated with the insulating oil.
More particularly, the invention relates to an electrical insulating oil and oil-filled electrical appliances in which the insulating oil comprises a mixture of alkylbiphenyl and/or alkylnaphthalene and monoolefin and/or diolefin having two aromatic nuclei. The electrical insulating oil of the invention is quite suitable for use in oil-filled electrical appliances in which insulating materials or dielectric materials made of plastics such as polyolefins are employed.
(2) Description of the Prior Art
Electrical appliances such as oil-filled capacitors, oil-filled power cables and transformers have recently been made to withstand high electric voltages while being small in size. With this tendency, various kinds of plastic films are used together with conventional insulating paper.
In the conventional art, refined mineral oils, polybutenes, alkylbenzenes, polychlorinated biphenyls and the like are used as electrical insulating oils; however, they have several drawbacks. For example, the use of polychlorinated biphenyls was discontinued because it constitutes a public health hazard that is characteristic of halogenated aromatic hydrocarbons. Furthermore, the conventional electrical insulating oils are not satisfactorily compatible with the foregoing plastic materials such as polyolefin films which are recently used in oil-filled electrical appliances.
With the requirements of high-voltage withstanding and size reduction, it is necessary that the electrical insulating oil has a high dielectric breakdown voltage, a low dielectric loss tangent, and good hydrogen gas absorbing capacity.
The hydrogen gas absorbing capacity indicates the stability of the insulating oil against corona discharge (partial discharge) under high electric voltage conditions. The higher the gas-absorbing capacity, the smaller the likelihood of corona discharge, which leads to the advantage of the insulating oil having excellent stability or durability.
Meanwhile, in order to meet the requirement of high-voltage use, plastic films such as polyolefin films, polystyrene films and polyester films are used to replace either partially or completely the conventional insulating paper as insulating materials or dielectric materials for electrical appliances such as oil-filled electric cables and capacitors. In view of their dielectric strength, dielectric loss tangent and dielectric constant, polyolefin films, especially polypropylene and cross-linked polyethylene films, are preferred as the plastic films.
When these polyolefin films are impregnated with insulating oils, some oils cause the films to swell to some extent. If a film becomes swollen, the thickness of the insulating layer increases. As a result, the resistance to the flow of insulating oil increases in electrical cables, and insufficient impregnation with insulating oil occurs in electric capacitors, causing the formation of voids (unimpregnated portions) and the undesirable lowering of the corona discharge voltage.
In connection with the above-mentioned conventional electrical insulating oils, the values of the dielectric breakdown voltages (BDV) and the dielectric loss tangents (tan .delta.) are satisfactory to a certain extent, but the hydrogen gas absorbing capacity or corona discharge characteristics and the stability of the dimensions of polypropylene films are not satisfactory.